On the surface of an object to be coated such as an automobile body, a plurality of coating films having various roles are formed sequentially, and thus the object to be coated is protected, as well as beautiful appearance and an excellent design. A common method for foaming such a plurality of coating films, e.g., for a steel plate, is a method in which an undercoating film such as an electrodeposition coating film is formed on an object to be coated that is excellent in conductivity, and then an intermediate coating film, a base coating film, and a clear coating film are formed thereon one after another.
In view of further request for reducing burden on the environment such as energy saving and reduction in CO2 emission, it is required to lower a heat curing temperature in a coating film formation. Moreover, in automobile manufacturing fields, further reduction in weight of an automobile body is required according to the development of electric vehicles. The reduction in weight of an automobile body brings about an improvement in fuel economy, and therefore, it is also effective in terms of energy saving and reduction in CO2 emission. One way to reduce the weight of an automobile body is to replace a steel plate part with a resin part.
In conventional coating for steel plates and resin members, it is common that different coating compositions are used respectively in consideration of the characteristics and softening temperature of each member. On the other hand, in coating of automobile bodies, for the purposes of simplifying coating step and coating management and improving hue consistency in a coated product, it has been demanded to commonize coating compositions to be used for coating various components. In the case of commonizing the coating composition for a steel plate and that for a resin member, however, it is necessary to adjust the curing temperature of the coating composition to a temperature lower than the conventional curing temperature in consideration of the heat resistance of the resin member. Moreover, in the case of forming a coating film on an object to be coated having both a steel plate part and a resin part, deformation may occur during heat curing due to a difference in thermal expansion coefficient between the members. Therefore, it is extremely important to lower the heat curing temperature and minimize the influence of the thermal history on each member in commonizing the coating composition.
On the other hand, lowering the heat curing temperature may reduce the crosslinking density of a resulting coating film and coating film performance such as water resistance and chipping resistance may deteriorate.
JP-A-2011-131135 (Patent Document 1) discloses a method for forming a multilayer coating film that involves applying an aqueous intermediate coating composition to a substrate having both a steel plate and a plastic substrate to form an intermediate coating film, then applying an aqueous base coating composition to the famed intermediate coating film to form a base coating film, then applying an organic solvent type clear coating composition to form a clear coating film, and heating and curing the three layers, namely, the intermediate coating film, the base coating film, and the clear coating film, in which the aqueous base coating composition contains (a) an acrylic resin emulsion, (b) a water-soluble acrylic resin, (c) a melamine resin, and (d) a propylene glycol monoalkyl ether. A multilayer coating film obtained by this forming method may not exhibit sufficient chipping resistance when being cured, for example, at a temperature of 100° C. or less.